This invention relates to a thermal imaging medium for the recordation of information. More particularly, it relates to a laminar imaging medium having improved resistance to stress-induced delamination.
The provision of images by resort to media which rely upon the generation of heat patterns has been well known. Thermally imageable media are particularly advantageous inasmuch as they can be imaged without certain of the requirements attending the use of silver halide based media, such as darkroom processing and protection against ambient light. Moreover, the use of thermal imaging materials avoids the requirements of handling and disposing of silver-containing and other processing streams or effluent materials typically associated with the processing of silver halide based imaging materials.
Various methods and systems for preparing thermally generated symbols, patterns or other images have been reported. Examples of these can be found in U.S. Pat. No. 2,616,961 (issued Nov. 4, 1952 to J. Groak); in U.S. Pat. No. 3,257,942 (issued June 28, 1966 to W. Ritzerfeld, et al.); in U.S. Pat. No. 3,396,401 (issued Aug. 6, 1968 to K. K. Nonomura); in U.S. Pat. No. 3,592,644 (issued July 13, 1971 to M. N. Vrancken, et al.); in U.S. Pat. No. 3,632,376 (issued Jan. 4, 1972 to D. A. Newman); in U.S. Pat. No. 3,924,041 (issued Dec. 2, 1975 to M. Miyayama, et al.); in U.S. Pat. No. 4,123,578 (issued Oct. 31, 1978 to K. J. Perrington, et al.); in U.S. Pat. No. 4,157,412 (issued June 5, 1979 to K. S. Deneau); in Great Britain Patent Specification 1,156,996 (published July 2, 1969 by Pitney-Bowes, Inc.); and in International Patent Application No. PCT/US87/03249 of M. R. Etzel (published June 16, 1988, as International Publication No. WO 88/04237).
In the production of a thermally actuatable imaging material, it may be desirable and preferred that an image-forming substance be confined between a pair of sheets in the form of a laminate. Laminar thermal imaging materials are, for example, described in the aforementioned U.S. Pat. Nos. 3,924,041 and 4,157,412 and in the aforementioned International Patent Application No. PCT/US87/03249. It will be appreciated that the sheet elements of a laminar medium will afford protection of the image-forming substance confined therebetween against the effects of abrasion, rub-off and other physical stimuli. In addition, a laminar medium can be handled as a unitary structure, thus, obviating the requirement of bringing the respective sheets of a two-sheet imaging medium into proper position in the printer or other apparatus used for thermal imaging of the medium material.
In a laminar thermal imaging medium comprising at least a layer of image-forming substance confined between a pair of sheets, image formation may depend upon preferential adhesion of the image-forming substance to one of the sheets. Typically, such a laminar medium material will be designed such that the image-forming substance will be preferentially adherent to one of the sheets, before thermal actuation of regions of the laminar medium, and preferentially adherent to the other sheet in actuated or "exposed" regions. Accordingly, separation of the sheets of the laminar medium material, in the case where there has been no thermal actuation or "exposure", provides a layer of image-forming substance on the one sheet to which it is preferentially adherent. Separation of the sheets, of the medium material, in the case where the medium is exposed to radiation over its entire area and sufficient in intensity to reverse the preferential adhesion, provides the layer of image-forming substance on the opposite sheet.
Inasmuch as a laminar thermal imaging medium of the aforedescribed type will be designed such that the image-forming substance is preferentially adherent to only one of the sheets before and until thermal actuation, the laminar medium material may exhibit an undesirable tendency to delaminate upon subjection to handling, cutting or other stress-inducing conditions or operations. For example, it may be desirable to form a laminar medium from a pair of endless sheet or web materials and to then cut, slit or otherwise provide therefrom individual film units of predetermined size. A reciprocal cutting and stamping operation used for the cutting of individual film units may create stress influences in the medium, causing the sheets to separate at the point of weakest lamination--typically, at the interface where, upon thermal actuation, the preferential adhesion of the image-forming substance would be reversed. Individual film-sized units cut from a web of laminar material may, during handling in a printer or imaging apparatus, or as a result of a user flexing or otherwise torturing the film unit, delaminate in an undesired and premature fashion.